1. Field of the Invention
The present invention relates to a steel cord of a three-layer twisted construction and a tire manufactured by using the steel cord.
2. Description of Related Art
Conventionally, various constructions of steel cord of this kind are known. The wires of the steel cord comprise 3+9+15, i.e., a core layer composed of three wires, an intermediate layer composed of nine wires, and an outer layer composed of 15 wires. The wires of each layer consist of filaments of the same diameter. Japanese Laid-Open Patent Publication No. 61-89390 discloses a steel cord including lapping wires, consisting of filaments of smaller diameter, arranged around the outer layer. Japanese Laid-Open Patent Publication No. 54-50640 disclose cord of 2+7+12 or 3+8+13 and free space ratio (the ratio of the wire-free space to the whole space including the wire-free space and the wire-occupied space, i.e. the ratio of a circumference including axes of wires in one layer to an area of the circumference not occupied by the wires) is in the range from 14% to 25%. Japanese Laid-Open Patent Publication No. 58-177702 discloses a steel cord having free space ratio of 0 to 33% in the core layer and 5 to 15% in the intermediate and the outer layers. Japanese Laid-Open Patent Publication No. 59-223503 discloses a steel cord in which the core layer, the intermediate layer, and the outer layer are composed of four wires, nine wires, and 14 wires, respectively.
However, the above-described conventional steel cords have disadvantages. For example, in the steel cord of 3+9+15, it is difficult for rubber to penetrate into the gaps between adjacent filaments. Consequently, filaments are rubbed against each other during the use of a tire. As a result, filaments are worn down or corroded. Thus, the steel cord becomes frail. According to the steel cord disclosed in Japanese Laid-Open Patent Publication No. 54-50640 having the free space ratio of 14% to 25% and the wire construction of 2+7+12 or 3+8+13, it is difficult for rubber to penetrate sufficiently into the inner side of the intermediate layer because the gaps between adjacent wires are small. In the wire construction of 2+7+12, the core layer is composed of two wires. Therefore, the use of a tire including the core layer causes the wires to be bent. As a result, directionality is generated in the rigidity of the wires and the arrangement of the steel cord is likely to be non-uniform.
The above-described Japanese Laid-Open Patent Publication No. 58-177702 also discloses a steel cord in which the core layer is composed of three wires, the intermediate layer is composed of eight wires, and the outer layer is composed of 13 wires and each wire consists of filaments of the same diameter. In this construction although rubber penetrates into the gap between the intermediate layer and the outer layer, it does not penetrate sufficiently into the gap between the intermediate layer and the core layer. As a result, it is likely that wires are corroded and deteriorate in quality.
In a steel cord in which the core layer is composed of four wires, the intermediate layer is composed of nine wires, and the outer layer is composed of 14 wires, it is difficult for rubber to penetrate from the intermediate layer into the core layer and the diameter of the steel cord is inevitably large. As a result, a rubber tends to be thicker to embed the steel cord therein, and consequently, the weight of a tire comprising the steel cord is necessarily great.